As disclosed in Japanese Unexamined Patent Application Publication No. H6-74968 and Japanese Unexamined Patent Application Publication No. 2000-81449 (which corresponds to U.S. Pat. No. 6,257,061), capacitive acceleration sensors are in wide use in vehicle control and the like. This type of acceleration sensor has a capacitor for detection in which the distance between electrode plates is variable. The acceleration sensor is so designed that the following operation is performed: the state of charge storage in the capacitor based on change in the distance between electrode plates due to acceleration application is subjected to charge-voltage conversion through a charge-voltage conversion circuit; it is thereby outputted as an acceleration signal.
With respect to capacitive acceleration sensors, recently, the following has been implemented to reduce their size and simplify assembling processes: an inertial displacement body for acceleration detection, the comb-like electrodes of a capacitor for detection, and the like are integrally formed by micromachining technology for silicon substrates. However, as sensor components are microminiaturized as mentioned above, problems become prone to occur. Such problems can include malfunctions due to breakage in a beam portion that couples an inertial displacement body with a frame, fine foreign particle lodged in between electrodes, and the like. The above prior arts disclose capacitive acceleration sensors provided with a self-diagnosing function for these malfunctions.
However, the technologies disclosed in the above prior arts have problems. In either technology, acceleration detection must be halted during self-diagnosis whereupon a simulated acceleration, pseudo-acceleration, false acceleration, or the like, is applied to a detecting capacitor of the sensor for testing purposes resulting in the application of the false acceleration for self-diagnosis on an intermittent or irregular basis and leading to attendant drawbacks. As an example, it will be assumed that the displacement rigidity of a movable electrode becomes abnormal due to breakage, lodging of foreign particle, or the like. In self-diagnosis mode, the movable electrodes are forcedly displaced; therefore, the anomaly is easily detected during the self diagnosis. In acceleration detection mode, however, effective displacement is not necessarily caused in the movable electrodes unless great acceleration is externally applied. Thus, if an anomaly occurs only during operation of the acceleration mode it is difficult to find.
When false acceleration is applied in self-diagnosis mode, the movable electrodes of the detecting capacitor are forcedly displaced from their neutral points. Therefore, when the mode is changed to acceleration detection mode in which false acceleration is not applied, the influence of forced displacement of the electrodes in self-diagnosis mode remains for a certain period like damped vibration. As a result, the zero point of a charge-voltage conversion circuit takes forever to stabilize, and thus a problem occurs. The accuracy of acceleration detection is degraded.
Thus, the sensor cannot monitor an anomaly of the sensor with high accuracy all the time, and further, the sensor cannot detect acceleration with high accuracy.
Further, recently, demand for higher-sensitivity acceleration sensors for vehicles has been increased. However, since a charge-voltage conversion circuit is used for the output portion of sensors, problems arise. With the enhancement of the sensitivity of sensors, the zero point can drift with time or due to temperature change because of the following: deterioration in or the temperature characteristics of operational amplifiers that play a key role in charge-voltage conversion or peripheral discrete elements.
Japanese Unexamined Patent Publication No. 2001-336618 discloses the following method as a solution for the above-mentioned problems: conditions for detecting the horizontal position of a vehicle are provided for automatically correcting errors due to the temperature or aging of an acceleration sensor installed in the vehicle. (The conditions include a transmission in neural, a brake released, and no rotation of wheels.) The acceleration sensor is corrected only when the conditions are met.
However, the method disclosed in the above prior art includes the following problems:
(1) The zero point cannot be corrected unless the vehicle is brought into a horizontal position; therefore, the timing of correction is limited.
(2) A signal from the vehicle is indispensable to acquire conditions for horizontality detection. For this reason, an acceleration sensor cannot automatically carry out correction by itself.
Thus, the sensor cannot compensate a zero point drift all the time, and the sensor cannot compensate the zero point drift without additional parts.